[clang] [-Wunsafe-buffer-usage] Improve null-termination analysis on conditionals (PR #176262)

[Ziqing Luo via cfe-commits]

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@@ -29,6 +29,20 @@ void f(int x, int y) {
   return;
 }
 
+// Test nested conditional expressions:
+void testNested(char * message) {
+  fprintf(stderr, "AssertMacros: %s", (0!=0) ? message : ((0!=0) ? message : ""));
+}
+
+// If the conditional cannot be constant-folded, try analyze both branches:
+void testConditionalAnalysis(char * message, int x) {
+  fprintf(stderr, "AssertMacros: %s", (x!=0) ? "hello" : "world");
+  fprintf(stderr, "AssertMacros: %s", (0!=0) ? message : ((x!=0) ? "hello" : "world"));
+  fprintf(stderr, "AssertMacros: %s", (x!=0) ? (((x!=0) ? "hello" : "world")) : ((x!=0) ? "hello" : "world"));
+  fprintf(stderr, "AssertMacros: %s", (x!=0) ? (((x!=0) ? "hello" : "world")) : ((x!=0) ? "hello" : message)); //\
+		cxx-warning{{function 'fprintf' is unsafe}} cxx-note{{string argument is not guaranteed to be null-terminated}}											       
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ziqingluo-90 wrote:

> Why is this raised only for C++ mode? I couldn't see any hints for that restriction.

This is a part of warning about unsafe libc functions under the C++ Safe Buffers project.  It does not make much sense to emit these warnings on C programs as they don't have safe alternatives.

https://github.com/llvm/llvm-project/pull/176262